Molecularly Mixed Composite Membranes: Challenges and Opportunities

Amidation-Reaction Strategy Constructs Versatile Mixed Matrix Composite Membranes towards Efficient Volatile Organic Compounds Adsorption and CO2 Separation

Mixed matrix composite membranes (MMCMs) have shown advantages in reducing VOCs and CO2 emissions. Suitable composite layer, substrate, and good compatibility between the filler and the matrix in the composite layer are critical issues in designing MMCMs. This work develops a high-performance UiO-66-NA@PDMS/MCE for VOCs adsorption and CO2 permea-selectivity, based on a simple and facile fabrication of composite layer using amidation-reaction approach on the substrate. The composite layer shows a continuous morphological appearance without interface voids. This outstanding compatibility interaction between UiO-66-NH2 and PDMS is confirmed by molecular simulations. The Si─O functional group and UiO-66-NH2 in the layer leads to improved VOCs adsorption via active sites, skeleton interaction, electrostatic interaction, and van der Waals force. The layer and ─CONH─ also facilitate CO2 transport. The MMCMs show strong four VOCs adsorption and high CO2 permeance of 276.5 GPU with a selectivity of 36.2. The existence of VOCs in UiO-66-NA@PDMS/MCE increases the polarity and fine-tunes the pore size of UiO-66-NH2, improving the affinity towards CO2 and thus promoting the permea-selectivity for CO2, which is further verified by GCMC and EMD methods. This work is expected to offer a facile composite layer manufacturing method for MMCMs with high VOC adsorption and CO2 permea-selectivity.

Metallic–Organic Cages (MOCs) with Heterometallic Character: Flexibility-Enhancing MOFs

The dichotomy between metal–organic frameworks (MOFs) and metal–organic cages (MOCs) opens up the research spectrum of two fields which, despite having similarities, both have their advantages and disadvantages. Due to the fact that they have cavities inside, they also have applicability in the porosity sector. Bloch and coworkers within this evolution from MOFs to MOCs manage to describe a MOC with a structure of Cu2 paddlewheel Cu4L4 (L = bis(pyrazolyl)methane) with high precision thanks to crystallographic analyses of X-ray diffraction and also SEM-EDX. Then, also at the same level of concreteness, they were able to find the self-assembly of Pd(II)Cl2 moieties on the available nitrogen donor atoms leading to a [Cu4(L(PdCl2))4] structure. Here, calculations of the DFT density functional allow us to reach an unusual precision given the magnitude and structural complexity, explaining how a pyrazole ring of each bis(pyprazolyl)methane ligand must rotate from an anti to a syn conformation, and a truncation of the MOC structure allows us to elucidate, in the absence of the MOC constraint and its packing in the crystal, that the rotation is almost barrierless, as well as also explain the relative stability of the different conformations, with the anti being the most stable conformation. Characterization calculations with Mayer bond orders (MBO) and noncovalent interaction (NCI) plots discern what is important in the interaction of this type of cage with PdCl2 moieties, also CuCl2 by analogy, as well as simple molecules of water, since the complex is stable in this solvent. However, the L ligand is proved to not have the ability to stabilize an H2O molecule.

Assembly of a Heterometallic Cu(II)-Pd(II) Cage by Post-assembly Metal Insertion

Porous structures based on multi-metallic motifs are receiving growing interest, but their general preparation still remains a challenge. Here, we report the self-assembly and structure of a Cu^II metal-organic cage (MOC) that is functionalized with free bis(pyrazolyl)methane sites. The homometallic Cu₄L₄ cage is isolated as a water-stable crystalline solid, and its formation is dependent on metal-ligand stoichiometry and the pre-organization of the Cu₂ paddlewheel. We show by X-ray diffraction and SEM-EDX that Pd^II chloride can be quantitatively inserted into the free chelating sites of the MOC to yield a [Cu₄(L(PdCl₂))₄] structure. Moreover, the solvent employed in the metalation dictates the solid-state isomerism of the heterometallic cage─a further handle to control the MOC's structural diversity and permanent porosity.

Highly Ion-Permselective Porous Organic Cage Membranes with Hierarchical Channels

Membranes of high ion permselectivity are significant for the separation of ion species at the subnanometer scale. Here, we report porous organic cage (i.e., CC3) membranes with hierarchical channels including discrete internal cavities and cage-aligned external cavities connected by subnanometer-sized windows. The windows of CC3 sieve monovalent ions from divalent ones and the dual nanometer-sized cavities provide pathways for fast ion transport with a flux of 1.0 mol m^-2 h^-1 and a mono-/divalent ion selectivity (e.g., K⁺/Mg²⁺) up to 10³, several orders of magnitude higher than the permselectivities of reported membranes. Molecular dynamics simulations illustrate the ion transport trajectory from the external to internal cavity via the CC3 window, where ions migrate in diverse hydration states following the energy barrier sequence of K⁺ < Na⁺ < Li⁺ ≪ Mg²⁺. This work sheds light on ion transport properties in porous organic cage channels of discrete frameworks and offers guidelines for developing membranes with hierarchical channels for efficient ion separation.

A smart and responsive crystalline porous organic cage membrane with switchable pore apertures for graded molecular sieving

Membranes with high selectivity offer an attractive route to molecular separations, where technologies such as distillation and chromatography are energy intensive. However, it remains challenging to fine tune the structure and porosity in membranes, particularly to separate molecules of similar size. Here, we report a process for producing composite membranes that comprise crystalline porous organic cage films fabricated by interfacial synthesis on a polyacrylonitrile support. These membranes exhibit ultrafast solvent permeance and high rejection of organic dyes with molecular weights over 600 g mol^-1. The crystalline cage film is dynamic, and its pore aperture can be switched in methanol to generate larger pores that provide increased methanol permeance and higher molecular weight cut-offs (1,400 g mol^-1). By varying the water/methanol ratio, the film can be switched between two phases that have different selectivities, such that a single, 'smart' crystalline membrane can perform graded molecular sieving. We exemplify this by separating three organic dyes in a single-stage, single-membrane process.

Supramolecular Polymer Network Membranes with Molecular-Sieving Nanocavities for Efficient Pre-Combustion CO2 Capture

Pre-combustion membrane CO₂ capture from syngas before utilizing the clean hydrogen fuel, demands very challenging membrane materials with simultaneous high thermal resistance, precise subnanometer size-selectivity, and robust processability. Here, an unconventional yet ultra-facile nanocomposite membrane design using 4-sulfocalix[4]arene (SCA4) molecules, a highly interactive member of soluble organic macrocyclic cavitands (OMCs) with a precise ≈3.0Å open cavity, is reported, to effectively sieve CO₂ (3.3Å) from H₂ (2.89Å). By simply infiltrating dissolved SCA4 molecules into prefabricated polymer membranes, they form extensive 3D supramolecular polymer networks (SPNs) with the polymer backbones through multi-site ionic interactions. Bearing distinctly molecular-sieving nanocavities, these otherwise amorphous SPN membranes deliver drastically enhanced mixed-gas H₂ /CO₂ separation under an industrial high-temperature-and-pressure environment with 4.35 times higher selectivity being achieved, allowing them to well outperform most existing polymer-based materials and even rival many state-of-the-art but delicate inorganic and framework-based membranes. They also demonstrate enhanced mechanical properties and long-term operation stability. Most attractively, the SPN membranes obtain a molecularly homogeneous, single-phase composite structure that can significantly surpass conventional phase-segregated mixed-matrix membranes in processability. Accompanied by the widely tunable OMC structures, this work can provide a versatile toolbox for designing advanced molecular-sieving membranes with an optimal balance of performance, robust properties, and scalability.

Tunable Supramolecular Cavities Molecularly Homogenized in Polymer Membranes for Ultraefficient Precombustion CO2 Capture

Processable molecular-sieving membranes are important materials for realizing energy-efficient precombustion CO₂ capture during industrial-scale hydrogen production. However, the promising design of mixed matrix membranes (MMMs) that aims to integrate the molecular-sieving properties of nanoporous architectures with industrial processable polymers still faces performance and fabrication issues due to the formation of segregated nanofiller domains in their polymer matrices. Here, an unconventional nanocomposite membrane design is proposed using soluble organic macrocyclic cavitands (OMCs) with tunable open cavity sizes that not only mitigate the formation the discrete nanofiller phases but also deliver distinct molecular-sieving separations. The versatile organic-solvent solubility coupled with highly interactive functionalities of OMCs allows them to obtain molecularly homogeneous mixing with matrix polymers and form only one integral continuous phase crucial to the robust processability of polymers. A series of polybenzimidazole-based molecularly mixed composite membranes (MMCMs) are fabricated via the incorporation of a soluble and thermally stable OMC choice, sulfocalixarenes, with various cavity sizes. These membranes achieve outstanding high-temperature mixed-gas H₂ /CO₂ separation performances comparable with several state-of-the-art molecular-sieving membranes owing to effective size-sieving gas passages through the open or partially-intruded supramolecular cavities. The broadly tunable structures and functionalities of OMCs would make their MMCMs attractive for other energy-intensive molecular separations.

Polyphenylsulfone (PPSU)-Based Copolymeric Membranes: Effects of Chemical Structure and Content on Gas Permeation and Separation

Although various polymer membrane materials have been applied to gas separation, there is a trade-off relationship between permeability and selectivity, limiting their wider applications. In this paper, the relationship between the gas permeation behavior of polyphenylsulfone(PPSU)-based materials and their chemical structure for gas separation has been systematically investigated. A PPSU homopolymer and three kinds of 3,3',5,5'-tetramethyl-4,4'-biphenol (TMBP)-based polyphenylsulfone (TMPPSf) copolymers were synthesized by controlling the TMBP content. As the TMPPSf content increases, the inter-molecular chain distance (or d-spacing value) increases. Data from positron annihilation life-time spectroscopy (PALS) indicate the copolymer with a higher TMPPSf content has a larger fractional free volume (FFV). The logarithm of their O₂, N₂, CO₂, and CH₄ permeability was found to increase linearly with an increase in TMPPSf content but decrease linearly with increasing 1/FFV. The enhanced permeability results from the increases in both sorption coefficient and gas diffusivity of copolymers. Interestingly, the gas permeability increases while the selectivity stays stable due to the presence of methyl groups in TMPPSf, which not only increases the free volume but also rigidifies the polymer chains. This study may provide a new strategy to break the trade-off law and increase the permeability of polymer materials largely.

Mixed Dimensional Nanostructure (UiO-66-Decorated MWCNT) as a Nanofiller in Mixed-Matrix Membranes for Enhanced CO2 /CH4 Separation

Mixed-matrix membranes (MMMs) with combination of two distinct dimensional nanofillers (such as 1D-3D, 2D-3D, or 3D-3D, etc.) have drawn special attention for gas separation applications due to their concerted effects on gas permeation and mechanical properties. An amine-functionalized 1D multiwalled carbon nanotube (NH₂ -MWCNT) with exceptional mechanical strength and rapid gas transport was crosslinked with an amine-functionalized 3D metal-organic framework (UiO-66-NH₂ ) with high CO₂ affinity in a Schiff base reaction. The resultant crosslinked mixed-dimensional nanostructure was used as a nanofiller in a polysulfone (PSf) polymer matrix to explore the underlying synergy between 1D and 3D nanostructures on the gas separation performance of MMMs. Cross-sectional scanning electron microscopy and mapping revealed the homogenous dispersion of UiO-66@MWCNT in the polymer matrix. The MMM containing 5.0 wt. % UiO-66@MWCNT demonstrated a superior permeability 8.3 Barrer as compared to the 4.2 Barrer of pure PSf membrane for CO₂ . Moreover, the selectivity (CO₂ /CH₄ ) of this MMM was enhanced to 39.5 from the 28.0 observed for pure PSf under similar conditions of pressure and temperature.

Enantioselective Mixed Matrix Membranes for Chiral Resolution

Most pharmaceuticals are stereoisomers that each enantiomer shows dramatically different biological activity. Therefore, the production of optically pure chemicals through sustainable and energy-efficient technology is one of the main objectives in the pharmaceutical industry. Membrane-based separation is a continuous process performed on a large scale that uses far less energy than the conventional thermal separation process. Enantioselective polymer membranes have been developed for chiral resolution of pharmaceuticals; however, it is difficult to generate sufficient enantiomeric excess (ee) with conventional polymers. This article describes a chiral resolution strategy using a composite structure of mixed matrix membrane that employs chiral fillers. We discuss several enantioselective fillers, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), zeolites, porous organic cages (POCs), and their potential use as chiral fillers in mixed matrix membranes. State-of-the-art enantioselective mixed matrix membranes (MMMs) and the future design consideration for highly efficient enantioselective MMMs are discussed.

Metal-Organic Material Polymer Coatings for Enhanced Gas Sorption Performance and Hydrolytic Stability under Humid Conditions

Physisorbent metal-organic materials (MOMs) have shown benchmark performance for highly selective CO₂ capture from bulk and trace gas mixtures. However, gas stream moisture can be detrimental to both adsorbent performance and hydrolytic stability. One of the most effective methods to solve this issue is to transform the adsorbent surface from hydrophilic to hydrophobic. Herein, we present a facile approach for coating MOMs with organic polymers to afford improved hydrophobicity and hydrolytic stability under humid conditions. The impact of gas stream moisture on CO₂ capture for the composite materials was found to be negligible under both bulk and trace CO₂ capture conditions with significant improvements in regeneration times and energy requirements.